Introduction to Differential Carriers


There are two types of differential carriers – open and traction enhancing. Most stock Land Rover differentials are a two pinion open carrier design along with most other makes of cars and light trucks. First, lets talk about open differentials. As mentioned earlier. When open differentials were originally designed, they were a huge technological improvement over what was available prior. On paved roads in good weather i.e. with equal traction, they work superbly. They smoothly, efficiently and effectively do everything a differential should do.

Open Carrier

A standard open carrier is comprised of 4 major parts, the carrier case, spider gears (also called pinion gears), side gears and finally the cross shaft, which the spider gears rotate on. They come in either 2 pinion or 4 pinion styles. A 4 pinion design has 2 cross shafts and 4 spider gears as compared to a two pinion design that has only two spider or pinion gears. A 4 pinion carrier is substantially more durable although they do not improve the actual performance of the vehicle. An open diff is an open diff whether it is really strong or not! Most upgraded traction enhancing differential carriers are also substantially more durable so this is an additional benefit.
So what’s the problem with open differentials? Absolutely nothing until you get in unequal traction situations like being in many off road situations! When this happens, an open differential actually works exactly opposite of how you would want a differential to function off road. It will send the power to the tire with the least amount of traction! Why is this? Quite simple, torque in a power train follows the path of least resistance, sort of like electricity. Think of it this way with a real world example. Say you are negotiating a very difficult obstacle and you stop with one of your tires completely off of the ground. When you attempt to move, which tire is easier to spin, the one in the air, with no weight on it and hence no traction? Or the one on the ground with all of the vehicle weight and hence some traction? The answer is obvious; the tire in the air and this is exactly what happens!

The solution to this is to install a traction enhancing differential carrier. The purpose of any traction enhancing differential is to counteract the physics of the torque flow discussed in the previous paragraph i.e. to prevent the torque from following the path of least resistance and end up spinning the wrong tire. Instead you want it to force or encourage it to go to the tire with more traction.

You have probably noticed that I keep referring to traction enhancing differentials instead of “lockers”. This is because there are several types of traction enhancing differentials of which a “locker” or fully locking differential is just one type.

Lets discuss the various types available. They fall into three categories – limited slips, fully locking versions and although not a specific type of differential – Electronic Traction Control.

The topic of the relative benefits of the different types of traction enhancing differential carriers is a very common one. But it is one of the most difficult to get accurate information especially on public forums. For some reason, many people seem to be very opinionated about one style compared to the other, hence you tend to get what we call “personal opinion masquerading as knowledge”. This even applies to some vendors, who tend to tell you one type of traction enhancing differential is “better” than others. In our opinion, all of them are very good differentials but like anything else there are advantages and disadvantages to all types. We can specifically discuss all of these to help you make the best decision possible. We use 4 criterion to help you decide which type of differential carrier best suits your needs. These are:

1) intended use of the vehicle
2) the specific vehicle itself
3) personal budget
4) personal preference

This discussion can be extensive and we would encourage you to call and discuss it personally. We are happy to spend as much time as you need to help you thoroughly understand this topic. We are known for our technical knowledge so we can give you accurate information and good information helps you to make good decisions. Good decisions ensures you get the best value for your money.

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Limited Slip Differential

A limited slip is a differential carrier that allows differential action but there is some resistance to it happening, hence forcing some torque bias to the appropriate tire i.e. the one with more traction. There is a limited amount of slippage that occurs so although they improve the off road performance, they may not maximize it.

There are several types of these including helical worm gear (Detroit Truetracs), spring loaded clutch packs (Positraction) and friction cones versions (Auburn). I’m not going to discuss the specifics here, primarily because at the present time, only Detroit Truetracs are available for Land Rover applications.

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Fully Locking Differentials

A fully locking differential is exactly that, a differential carrier that prevents any differential action from occurring at all. This will maximize the traction available because 100% of the torque going into an axle assembly can go to the tire with the most amount of traction.

There are several categories of locking differentials – automatic, selectable and constant lockers.

An example of an automatic locking differential carrier is a Detroit Locker. By design, they are locked but they have the ability to provide differential action when needed, such as negotiating a turn on road.

An example of selectable locking differential carrier is an ARB Air Locker. A selectable locking differential is exactly that i.e. you can select when you want it engaged or disengaged. The selection can be accomplished several ways such as positive air pressure, vacuum or cable.

A spool is an example of a constantly locked differential. In some respects, calling it a differential is a stretch because they cannot provide any differential action at all. When in use any differential action that occurs is actually provided by tire slippage.

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Electronic Traction Control (ELC)

Electronic traction control is not a specific type of differential carrier although it is a traction enhancing system so warrants mentioning within this discussion. This is for several reasons:

1) Several models of Land Rovers now come equipped with this system.
2) It is trying to accomplish the same thing as a traction enhancing differential carrier.
3) ETC has some durability implications upon your existing stock differential carrier.
4) Question concerning its compatibility with actual traction enhancing differentials.

So how does it work? If you think back to the explanation of the torque flow in a power train i.e. the torque essentially following the path of least resistance. The result of which is that a tire that loses traction starts to speed up because there is less resistance/traction, to prevent it from happening. When this happens the ABS brake sensor, yes the same one that controls the ABS brakes, will detect this. It sends this information back to the computer and when it goes on for long enough and at a great enough differential in speed, it instructs the caliper to slow down that tire by pulsing the pads against the rotor and at a very high frequency i.e. 30 times/second! It is essentially preventing that tire from running away by putting on the brakes. By doing this it forces the differential to maintain some torque biasing.

In essence, ETC is a passive system that sits there until it detects a difference in wheel speed either left or right, or driveshaft speed, front to rear. So, is it compatible with traction enhancing differential carriers? Absolutely – use this example, assume you have fully locking differentials in both your front and rear differentials and you have a lockable differential in your transfer case and then you lock them all. At this point, physically it is impossible for any tire to spin at a different speed from any other tire, so essentially traction control is passively sitting there, happy as a clam because everything it is concerned with, is not/cannot spin at different speeds. Why is this even a question – because when the system was originally released, Land Rover officially pronounced that any modifications to the vehicle that impacted traction control, would void your warranty. That may have been true from a legal standpoint but it had nothing to do with the compatibility of ETC and aftermarket traction enhancing differentials and/or center differential locking kits. Rather it concerned internal warranty procedures and fine print. Once the vehicles warranty is expired this is no longer a concern.

There are some durability considerations with ETC systems. The pulsing of the brake caliper transmits itself down the axle shaft into the differential carrier. This pulsing causes the side and spider gears to bang together in an un-natural way, eventually causing premature failure of these parts in what can be a surprisingly short amount of time – about 45 minutes by our calculations and observations! There are advantages to an ETC equipped vehicle but it is not a system designed for long term aggressive off road usage. Luckily, this issue can be resolved by upgrading the differential carrier to a more durable aftermarket variety, such as we have discussed here.

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